(1) Field of the Invention
The present invention relates to radio frequency (RF) transmission in portable telecommunication devices and specifically to power amplifiers for amplifying RF signals for transmission.
(2) Description of Related Art
As portable telecommunication devices are developed, the number of frequency bands in usage is increasing rapidly. Many 3G standards have been developed, characterised by ever-increasing numbers of frequency bands. The number of frequency bands is further increasing as 4G standards are developed and implemented.
Components for portable telecommunication devices are typically designed to be capable of operation in multiple different frequency bands. This is to allow operation in the appropriate frequency band anywhere in the world, as well as to achieve savings in design and manufacture by utilising a common design.
The front end of such devices includes a power amplifier for amplifying an RF signal to provide sufficient power for transmission. Such power amplifiers are intrinsically narrow-band devices. The need to generate power from a power supply, such as a battery, that provides a relatively small DC voltage leads to the use of semiconductor devices of relatively high capacitance. Such a combination of a small supply voltage and a high capacitance leads to the power amplifiers being of low impedance and high Q-factor, thus limiting the bandwidth. The present invention is concerned with designs of power amplifiers that maximise the bandwidth of the power amplifier.
Another issue with power amplifiers is efficiency, because power amplifiers contribute a significant proportion of the power consumption of a mobile telecommunications device. This is particularly the case with the complex modulation schemes used in 3G/4G standards to increase data rates. Such schemes require linear power amplification that compromises overall system efficiency, for example requiring power amplifiers to be backed off from saturated maximum power.
In order to improve the bandwidth performance of a power amplifier, one approach is to increase the DC voltage used for driving the power amplifier. The voltage applied across an individual semiconductor device is limited by the physical material properties of the semiconductor as high voltages can cause breakdown.
Clifton et al., “Novel Multimode JpHEMT Front-end Architecture With Power-Control Scheme For Maximum Effeciency”, IEEE Transactions on Microwave Theory and Techniques, Vol. 53, No. 6, June 2005 discloses a power amplifier performed by a series stack of separate JPHEMT devices. The same RF input signal is supplied in parallel to the input terminal of each JPHEMT device, and the output terminals of the JPHEMT devices are connected in series to provide the amplified RF output signal. Due to the arrangement of the JPHEMT devices in a series stack, each one is subjected to a fraction of the overall DC bias voltage. Thus, breakdown of the semiconductor material in the individual JPHEMT devices is avoided.
Nonetheless, the power amplifier may be operated with a relative high overall DC supply voltage which provides a number of advantages, as follows. The increased voltage improves the bandwidth of the power amplifier, as compared to using a single JPHEMT device. In practical systems, this allows a single power amplifier to cover an increased number of RF bands, thereby reducing the number of power amplifiers required in a multi-band telecommunications device. The design also improves the efficiency by providing a higher gain and allowing operation further from the knee voltage. The power amplifier may also be implemented in a reduced area of semiconductor because the higher voltage operation improves the output power per unit area of semiconductor geometry. For example, the overall gate width of the power amplifier disclosed in Clifton et al. is 7.2 mm as compared to a gate width of 18 mm for a conventional 3V power amplifier.
Portable telecommunication devices typically operate from batteries having an output voltage that is constrained by the battery technology. The output voltage will also have a discharge characteristic in which the output voltage reduces over the discharge of the battery. In view of these constraints, a nominal DC power supply voltage in a portable telecommunications device might be of the order of 3V. In order to provide a higher voltage than is available from the battery, Clifton et al. discloses the use of a DC-DC converter to provide an increased DC bias voltage supply for the power amplifier.
Another issue in portable telecommunication devices is the discharge characteristic of batteries typically used as a power supply. In particular, the output voltage of the battery reduces non-linearly over the discharge of the battery, for example along a characteristic from 4.2V to 2.5V. Where the DC bias voltage is fixed, this issue may be dealt with by designing components to operate at a nominal bias voltage, for example 3.0V. When the actual output voltage of the battery is above the nominal output voltage, then the efficiency of the components is compromised, because the power derived from the actual DC bias voltage in excess of the nominal output voltage is effectively wasted. This impacts on the power consumption. Conversely, there is an impact on battery life as the battery may be considered to be discharged when the output voltage falls below the nominal bias voltage. Accordingly, the nominal bias voltage is typically set to provide a compromise between these two factors, for example at 3.0 or 3.5V.